This invention relates to a flame-type atomic absorption spectrophotometer adapted to atomize a sample by introducing an atomized sample liquid into a flame.
It is necessary with an atomic absorption spectrophotometer to convert a sample into an atomic vapor. Methods of atomization include the flame method which makes use of a chemical flame and the flameless method which does not use a chemical flame. A flame-type atomic absorption spectrophotometer forms a flame by mixing together inside a chamber a fuel gas and a combustion support gas which are supplied separately. This mixture is then caused to flow out of the slit opening of a burner and to undergo combustion. The kinds of the fuel gas and the combustion support gas, the flow rate of the fuel gas and the height (or the vertical position) of the burner are variously determined, depending on the target element to be analyzed. In other words, these parameters for a flame-type atomic absorption spectrophotometer must be changed appropriately according to the kind of element to be measured. Although acetylene (C2H2) and air are most frequently used as the fuel gas and the combustion support gas, respectively, nitrous oxide (N2O) is sometimes used as the combustion support gas when an element which forms a strong oxide in the flame such as aluminum and titanium is to be analyzed, because N2O is capable of forming a flame which is hotter and more strongly reductive.
Since the mixture of nitrous oxide and acetylene has a very high speed of combustion, there may be a backfire if it is directly ignited or the supply of both gases is stopped simultaneously when the flame is extinguished. Thus, for using a flame of nitrous oxide-acetylene gas, it has been known to initially supply a mixture of air and acetylene to the burner to ignite and then to increase the flow rate of the acetylene and thereafter to replace air with nitrous oxide. For effecting this ignition procedure, it has been known (as described in Japanese Patent Publication Tokko 60-22291) to check the increase in the flow rate of acetylene by detecting an increase in the quantity of emitted light from the flame by means of an optical sensor or the like. According to the prior method of carrying out this procedure, it was concluded that the flow rate of acetylene has sufficiently increased when the detected light intensity value has exceeded a predetermined reference value. If the initial flow rate of the air-acetylene mixture is large, and especially if the flow rate of acetylene is large, however, the light intensity is already large during the initial period and it may have exceeded the predetermined reference value. In such a situation, a further increase in the flow rate of acetylene cannot be detected and the switch-over to nitrous oxide cannot be effected.
It is therefore an object of this invention in view of such a situation to provide an improved flame-type atomic absorption spectrophotometer which, when using nitrous oxide as the combustion support gas, can dependably carry out the switching from an air-acetylene mixture to a nitrous oxide-acetylene mixture at the time of ignition.
A flame-type atomic absorption spectrophotometer embodying this invention, with which the above and other objects can be accomplished, may be characterized as comprising an optical sensor for detecting the quantity of light from its flame, a gas flow rate control device for independently controlling flows of a fuel gas, a first combustion support gas and a second combustion support gas which are supplied to a burner, and a combustion control unit for controlling the gas flow rate control device to stop supplying the first combustion support gas and to start supplying the second combustion support gas instead to the burner for the flame, based on how much the quantity of light detected by the optical sensor changes between the time of ignition and after the flow rate is changed for the fuel gas and/or the first combustion support gas.
The present invention is addressed to such a situation where the flame must be initially formed by usuing a first combustion support gas with a slower combustion speed such as air because the combustion speed of the second combustion support gas such as nitrous oxide is too fast. First, a mixture of the fuel gas and the first combustion support gas is supplied to the burner at a specified rate and ignited. The quantity of light from the flame at this moment is measured and stored in a memory as the initial value.
Prior to switching to the second combustion support gas with a faster combustion speed, the flow rate of the fuel gas is increased. As the flow rate of the fuel gas is increased, the control unit compares the detected quantity of light from the flame with the initial value stored in the memory. If the difference is less than a specified reference value, the control unit keeps increaing the flow rate. As soon as the diffence reaches the reference value, the first combustion support gas is replaced by the second combustion support gas. In this manner, the switching of the combustion support gas can be accomplished without the danger of a backfire.
Depending on the situation, the contol unit may also change the flow rate of the first combustion support gas.